Download CAPTURING STARLIGHT

Capturing Starlight
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Telescopes are used to collect electromagnetic radiation; a variety of devices are used to
record that radiation. Producing objective, repeatable data is fundamental to scientific
inquiry.
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Photography was the original recording medium, used from the 1860s to the late1980s.
Visible and infrared light is recorded chemically, on glass or film. Unlike your eyes,
photographic cameras are integrating devices, building up images over many minutes or
hours. Extremely faint celestial objects can be identified and explored.
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Today, images are recorded using digital electronic devices, known as CCD cameras.
CCDs convert light into electronic signals via the photoelectric effect. Images are stored
on computers in digital form. Most research-grade CCDs are cryogenically-cooled to
extremely low temperatures to improve their performance.
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Spectrographs are used to disperse light from celestial sources into a spectrum; either a
glass prism or a diffraction grating serves as the dispersing element. Here too, the light is
recorded digitally, using CCD cameras, and stored on a computer.
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All astronomical observations today are obtained and processed under computer control.
Digital images are ideally suited for manipulation with computers. The technical art of
image processing turns raw images into new information about the cosmos. Image
processing techniques transcend astronomy; they are used in medicine, geology, earth
resource analysis, and reconnaissance.
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Very high resolution astronomical images are created using the technique of
interferometry: the use of multiple telescopes to observe the same object simultaneously.
Computers process the combined signals from these telescopes to form a single detailed
image of celestial objects. Interferometry is most commonly used at radio wavelengths,
recently this technique has been extended to infrared and optical parts of the spectrum.

The Earth’s atmosphere strongly affects astronomical observations, limiting our ability to
produce clear, sharp images from the ground.
(a) Starlight is smeared out, due to the random motions of air. This effect is known as
seeing. The atmosphere limits the resolution of telescopes to about 1 arc second.
Twinkling starlight is an indicator of poor seeing, implying very blurred images.
(b) During cloudy nights, the sky can be blocked out completely.
(c) The atmosphere absorbs many segments of the electromagnetic spectrum, permitting
only radio waves, visible light, and small pieces of the infrared spectrum to reach Earth’s
surface.
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The blurring effects of Earth’s atmospheric motions can be partially offset using the techniques of
active and adaptive optics, in which telescope mirrors are adjusted quickly to correct for these
motions. The result is sharper images from ground-based telescopes.
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To observe those parts of the EM spectrum which do not pass through the atmosphere, spacebased telescopes are required.